1. Field of the Invention
The present invention relates to a variable attenuator for attenuating an optical signal transmitted between an optical signal source and an optical signal receiver. More specifically, the present invention relates to the reflection of a transmitted optical signal off of divided surfaces for variably attenuating the optical signal.
2. Description of the Related Art
In optical data communications, signals are typically transmitted from a signal source to a signal receiver over an optical fiber network. FIG. 1 illustrates the general concept of optical signal transmission between an optical signal source 5 and an optical signal receiver 10, using a high reflectivity (HR) coated surface 15. For the sake of simplicity the various light beams illustrated in the figures are all shown as arcs to help in distinguishing their direction of travel; this illustration should not be considered as indicating any particular characteristic of the light beams themselves.
Suppose light is introduced into the system through the optical signal source 5 (e.g., a single mode optical fiber). As the light exits the end of the optical signal source 5 it starts to spread out to form the "sending beam" 7. Sending beam 7 is illustrated as a series of solid arcs moving from the top of FIG. 1 to the bottom of FIG. 1. Sending beam 7 is collimated by a lens 20 (or other focusing means) and then it falls upon the HR coated surface 15.
The reflection of the sending beam 7 by the HR coated surface 15 is a "returning beam" 12 that travels to optical signal receiver 10 (e.g., a single mode fiber). The returning beam 12 is illustrated as a series of dotted-line arcs moving from the bottom of FIG. 1 to the top of FIG. 1. Returning beam 12 is refocused (by the same lens 20 as used for sending beam 7 or by a different focusing means, such as a separate lens) to be collected by optical signal receiver 10.
It is well known that if the HR coated surface 15 is a nearly flat, highly reflecting surface, the optical coupling from the optical signal source 5 to the optical signal receiver 10 will be very good, less than 0.5 dB loss in typical implementations using active alignment in manufacture. Further, it is well understood that if the reflecting surface of HR coated surface 15 is translated left or right by a few microns, the optical coupling will be changed negligibly.
Optical signal systems have a signal intensity range in which they function best. If a signal falls below the operational range, the system will either incorrectly detect the signal or will not detect the signal at all. If the signal is above the operational range, the system will saturate and may result in a false reading of the data in the optical signal. Thus, optical signal levels which are too high or too low result in unreliable transmission of data or can interfere with other data-carrying signals.
The path attenuation of a fiber is a function of fiber length and the fiber attenuation coefficient. Further, the sensitivity of the receiver and the emitter output may exhibit changes due to aging. Thus, many optical transmission lines are designed with built-in attenuators which attenuate the optical signals within the waveguide to be within the optimal functional range of the optical system.
There are several known ways of providing attenuation of an optical signal. One method involves the use Faraday rotation in suitable doped Garnet films. By varying the applied magnetic field from an electromagnet, the polarization of transmitted light is changed and by using polarization selective optical elements, the attenuation can be varied. A problem with this attenuation method is that the electromagnet dissipates large amounts of electrical power and is quite large.
Another known method of attenuation involves the use of motorized variable attenuators where, for example, an opaque attenuating wedge is driven into the beam path to block a portion of the optical signal beam. In addition to being bulky, however, this method also is costly and slow-acting.
An additional attenuation method involves the use of liquid crystal designs which can work at very low electrical power levels and which function in a manner similar to Faraday rotation, but with liquid crystal rotation of polarization. Such systems are temperature and polarization sensitive and organic material in the beam path can be chemically unstable, causing shortened device life.
Attenuation using Micro Electro Mechanical Systems (MEMS) technology has been accomplished using a Mechanical AntiReflection Switch (MARS) modulator, an example of which is illustrated in FIGS. 2 and 3. These devices operate on the principle that varying the phase between two portions of a light beam allows the attenuation of the optical signal to be controlled, as described in more detail below. FIG. 2 shows a cross-section of a typical MARS modulator, and FIG. 3 is a top view of the MARS modulator depicted in FIG. 2. A typical MARS modulator 50 has a conductive or semi-conductive based substrate 52 that is transparent to the operating optical band width of the modulator.
A membrane 54 is suspended above the substrate 52, thereby defining an air gap 56 in between the substrate 52 and the membrane 54. A membrane 54 is typically fabricated from a silicon nitride film which is a dielectric. A metal film 58 is deposited around the top periphery of the membrane 54. Since the metal film 58 is optically opaque, only the center 60 of the membrane 54 remains optically active. When an electrostatic potential is applied in between the metal film 58 and the below lying substrate 52, the metal film 58 becomes charged and is deflected by electrostatic forces toward the substrate 52. The result is that the membrane 58 deflects dowardwardly in the direction of arrows 59 and the size of the air gap 56 is reduced. By applying a potential difference of about 40 volts to electrical connections coupled to the membrane 54 and the substrate 52, large electric fields are developed between the substrate 52 and metal film 58 causing an electrostatic force between the membrane 54 and the underlying silicon large enough to bow the membrane 54 closer to the underlying silicon. By increasing the applied voltage, the cavity width is decreased. By varying the cavity width, the relative phase between light reflected by the membrane 54 and light reflected by the underlying substrate 52 is also varied, thereby allowing control of the attenuation.
In order to assemble the device and in order to equalize the gas pressure on each side of the membrane 54, and allow quick response time, it is necessary to perforate the membrane 54 with very small holes. In FIG. 3 the perforation of the membrane 54 with very small holes 62 is depicted. The membrane 54 has a natural mechanical resonance; the resonance is damped by the gas viscosity passing through the holes 62. The inclusion of the holes 62 in the membrane 54 results in an optical loss, but the size and number of the holes 62 is selected to minimize this optical loss to a negligible level. Typically such holes 62 are approximately 3-5 .mu.M in diameter and are provided merely to minimize vibration, i.e., they do not provide any optical functions.
FIG. 4 is a partial cross-sectional view of the prior art MARS modulator of FIG. 2. Light traveling from top to bottom, identified as 64 in FIG. 4, will be partially reflected by the membrane 54 and partially transmitted beyond the membrane 54. The partially reflected light is identified as 66 in FIG. 4. The light transmitted beyond the membrane 54 is reflected by the floor of the cavity; this reflected light is identified as 68 in FIG. 4. Depending upon the cavity width and the wavelength of light used, the reflections will interfere constructively or destructively when they are received by an optical receiver (not shown). Constructive interference occurs when the wavelengths of the two reflected signals are in sync with each other, thereby enhancing the strength or power of the returned signal, i.e., the signal is not attenuated. Destructive interference refers to the effect caused by the receipt at the light collector of the two reflected signals in an "out of sync" state, which results in a signal of lesser strength or power, i.e., an attenuated signal. Thus, by varying the cavity width, the attenuation of the optical signal can be increased or decreased selectively.
The cavity widths for maximum total reflectivity and for minimum total reflectivity differ by 1/4 wavelength. Thus, by applying a suitable change of voltage between the metal film 58 and the substrate 52, the membrane 58 can be moved from one extreme in reflectivity to the other, thus passing through the total range of possible attenuations by moving only about 0.4 microns (for radiation at 1545 nm). Further degradation in performance (e.g., high attenuation occurring at the minimum attenuation point) is likely to occur where there are membrane holes in the optical path, although the degradation is negligible.